This seems ridiculous that they can't change the flights for July 4th which is over a week away. I'd take the other posters advice about seeing if the ticket agent at the airport could help you. There is a very big difference between an 11:55am and 7:30am flight (namely sleep and/or what you get to do on your last night of vaca). If there are seats available on the later flight they should be your friends.

Sean